The present invention relates to an antenna transmission and reception system in a mobile communication system, and in particular, relates to an antenna transmission and reception system in which a large amount of data can be transmitted with high efficiency in each cell.
In future mobile communication cellular systems, an increased traffic of high-rate packet data is expected, particularly, in down links. Accordingly, these systems require the immediate development of wireless transmission methods capable of transmitting a large amount of data with high efficiency. For this development, the following method is currently being studied. This method gives high priority to users in good transmission environments using multilevel modulation methods to realize high-speed data transmission. In addition, array antenna technology improves the signal to interference noise ratio (SINR) of a received signal using space domain multiple access. Therefore, the adoption of array antenna technology is also expected.
In mobile communication systems, the requirements for reduction in the size and weight of mobile stations (portable terminals) are strict. Therefore, directional-beam transmission control would be effective when used in mobile communication systems. According to this control, a plurality of antennas are arranged on a base station which can use comparatively complicated signal processing so as to narrow the antenna directivity toward a particular mobile station. The directional-beam transmission control allows amplification of a desired signal and does not cause interference with other mobile stations. Thereby, high-quality communication can be realized and the available range of cells can be increased. On the other hand, since a few (two to four) antennas can be provided for each mobile station, the characteristics of the mobile communication system using spatial diversity reception can be effectively improved.
Now, description will be made of an example of an arrangement of a related antenna transmission and reception system with reference to FIG. 1.
In the related antenna transmission and reception system, each base station includes a transmitter having N (N is an integer of 2 or more) antennas and each mobile station includes a receiver having M (M is an integer of 2 or more) antennas. The base station performs directional-beam transmission while the mobile station performs diversity reception. The number of antennas N of the base station can be larger than the number of antennas M of the mobile station. Generally, N is set to 6 to 8 while and M is set to 2.
In FIG. 1, a transmitter of the base station (a) comprises an encoder 101, a modulator 102, a beam former 103, and transmission antennas 104-1 to 104-N. The encoder 101 receives a data signal and performs error correction coding. The modulator 102 divides an encoded bit string into segments each having a predetermined length and maps the segments to respective modulation symbols.
Referring to FIGS. 2A and 2B, description will be made of examples of a digitally modulated signal serving as an output of the modulator 102. FIG. 2A shows a quadrature phase shift keying (QPSK) signal in which each modulation symbol comprises encoded data consisting of two bits. FIG. 2B shows a 16 quadrature amplitude modulation (16QAM) signal in which each modulation symbol comprises encoded data consisting of four bits. Generally, the respective bits are mapped so as to reduce the bit error rate even when modulation symbols are mistaken for the adjacent modulation symbols (Gray code mapping).
Referring to FIG. 3, the beam former 103 assigns weights to the digitally modulated signal to thereby form directional beams. Herein, it is to be noted that the weights are complex. The output of the modulator 102 is divided into N signals. N denotes the number of transmission antennas. Multipliers 111-1 to 111-N multiply the respective signals by antenna weights. The transmission antennas 104-1 to 104-N transmit the respective antenna-weighted signals. Methods of calculating the antenna weights include a method based on estimation of the arrival angles of multipaths and a method utilizing adaptive algorithm control.
FIG. 4 shows an example of the geometrical arrangement of the transmission antennas. The interval between each of the N transmission antennas serving as an antenna array is set to be narrow in order to exhibit directivity. Usually, a wavelength interval of 0.5 is selected.
Turning back to FIG. 1, the receiver (b) comprises reception antennas 105-1 to 105-M, a diversity demodulator 106, and a decoder 107. The reception antennas 105-1 to 105-M receive transmission signals which independently experience fading in respective transmission paths (also referred to as channels). The mobile station generally receives scattered waves from all directions. Although interval between the reception antennas 105-1 to 105-M is narrower than that between the antennas of the base station, each reception antenna can receive an independently faded signal.
Referring to FIG. 5, the diversity demodulator 106 combines the signals received by the respective antennas at the maximum ratio and demodulates respective bits. Therefore, each of channel estimation units 121-1 to 121-M estimates the amplitude and phase of the received signal of the corresponding antenna. Complex conjugate operation units 122-1 to 122-M each calculate the complex conjugate of the corresponding channel estimation. Multipliers 123-1 to 123-M each multiply the corresponding received signal by the complex conjugate of the corresponding channel estimation, demodulate the phase of the received signal, and weight the amplitude of the signal in order to combine the received signals at the maximum ratio.
A combiner 124 adds the weighted antenna signals. A soft decision unit 125 performs soft-decision modulation of the respective bits in the phase-corrected modulation symbols. In the case of a QPSK signal, the component of the I axis (real axis) and that of the Q axis (imaginary axis) of a complex signal serving as an output of the combiner 124 can be used as soft-decision demodulation signals of first and second bits, respectively. In the case of an 8PSK or 8QAM signal, another means is required in order to obtain a soft-decision demodulation signal. Therefore, maximum likelihood estimation is generally used.
A demodulation bit is represented by i. The likelihood function of the demodulation bit is represented by Λ(i).
The likelihood function thereof is represented by:
                              Λ          ⁡                      (                                          b                ⋒                            i                        )                          =                                            min                              x                ⁢                                                                                              b                      i                                        =                    0                                                                        ⁢                                                                                                                      A                      p                                        ⁢                    r                                    -                                      x                    ⁢                                                                  ∑                                                  j                          =                          1                                                M                                            ⁢                                                                                          ⁢                                                                                                                              h                            j                                                                                                    2                                                                                                                        2                                -                                    min                              x                ⁢                                                                                              b                      i                                        =                    1                                                                        ⁢                                                                                                                      A                      p                                        ⁢                    r                                    -                                      x                    ⁢                                                                  ∑                                                  j                          =                          1                                                M                                            ⁢                                                                                          ⁢                                                                                                                              h                            j                                                                                                    2                                                                                                                        2                                                          (        1        )            where, Ap denotes the level of a pilot signal which is used for channel estimation, r denotes an output of the combiner 124, x denotes the modulation symbol of a modulation signal, and hj denotes a channel estimation of the antenna received signal. The channel estimation is used to adjust the levels of Apr and x.
The soft-decision modulation operation shown by the above expression (1) will now be described in brief with reference to FIG. 6. Herein, FIG. 6 shows a 16QAM signal. The demodulation of a first bit will now be described. First, it is assumed that a bit 0 is transmitted. The square of the distance between each modulation symbol and r is calculated. Then, the minimum value of the squares is obtained. In FIG. 6, the symbol indicating 0001 has the minimum value. Subsequently, it is assumed that a bit 1 is transmitted. The square of the distance between each modulation symbol and r is calculated. Then, the minimum value of the squares is obtained. In FIG. 6, the symbol indicating 1001 has the minimum value. The difference between the minimum values serves as a soft-decision demodulation signal of the first bit. In the case shown in FIG. 6, the difference indicates a negative value, thus resulting in modulation of the bit having a value of approximately zero (zero in hard decision).
Second to fourth bits are sequentially subjected to similar soft-decision demodulation. Thus, soft-decision demodulation signals of the second to fourth bits can be obtained. The decoder 107 performs error correction decoding using the soft-decision demodulation signals of the respective bits obtained by the above maximum likelihood estimation. Convolution coding/Viterbi decoding and turbo coding/decoding are often used as error correction methods.
The related antenna transmission and reception system illustrated in FIG. 1 exhibits excellent performance in a large cell system including base stations having high antennas. Herein, it is noted that the high antennas mean that the height of each antenna is high. On the contrary, low antennas mean that the height of each antenna is low. In the large cell system, since the distance between each base station and each mobile station is large, the difference between the arrival angles of multipaths is small. Beams are narrowed using the directional-beam transmission control, thus obtaining a large gain.
Referring to FIG. 7, description will be made of the beam-gain characteristics change depending on the number of antennas. In a case where the arrival angles of transmission paths are concentrated in the direction A (0°), the number of antennas is increased to narrow the beams. As a consequence, the peak gain is increased. The peak gain in transmission with six antennas (hereinbelow, refer to as 6-antenna transmission) is higher that in transmission with three antennas (hereinbelow, refer to as 3-antenna transmission) by 3 dB. However, in a small-to-medium-sized cell system including base stations having low antennas, each base station is close to each mobile station. Therefore, the difference between the arrival angles of the paths is increased.
For example, it is assumed that the arrival paths are not only in the direction A (0°) but also in the directions B and C (±15°). In the case of the 6-antenna transmission, the gain in each of the directions B and C is decreased by about 10 dB. Accordingly, it is substantially impossible to transmit enough power to mobile stations through these paths. Therefore, the total transmission efficiency is reduced to about 40%. On the other hand, in the case of the 3-antenna transmission, although the gain in the direction A is reduced by 3 dB, the reduction in the beam gain in each of the directions B and C is small. The total transmission efficiency thereof is substantially 40%, or the same as the 6-antenna transmission case.
As described above, in the cell propagation environment having a large difference between the path arrival angles, even when the number of antennas is increased to narrow the directional beams to be transmitted, the transmission efficiency cannot be increased. On the other hand, when the mobile station is close to the base station, the signal correlation between antennas is decreased. Thus, it is difficult to form beams. Disadvantageously, the beam gain is also reduced.
According to MIMO (Multiple Input Multiple Output), a plurality of transmission antennas transmit different data in parallel and a plurality of reception antennas receive the data, respectively, thus realizing a high bit rate. This technique has been proposed and examined by H. Huang, M. Sandell, and H. Viswanathan, “Achieving high data rate on the UMTS downlink shared channel using multiple antennas”, pp. 372–377, 3G Mobile Communication Technologies, 26–28 Mar. 2001; and Ishii, Yoshida, and Atokawa, “Performance Comparison between Beamforming and MIMO in W-CDMA HSPDA”, B-5-111, Proceedings of the 2002 IEICE General Conference.